Modelagem molecular aplicada a reações de produção do biodiesel

Abstract

The transesterification reactions have been used to produce biodiesel from triglycerides found in vegetable oils and animal fat. Another important reaction is the esterification, which takes advantage of raw materials containing high levels of free fatty acids to produce esters (biodiesel). Biodiesel replaces fully or partially the petro-diesel and it is a renewable product. However, the thermodynamics of the biodiesel production reactions need to be investigated. The present work aims to study the esterification and transesterification reactions, two important routes for obtaining biodiesel. Thermodynamic properties were estimated, such as enthalpies of formation and reaction, Gibbs free energy and the equilibrium constants of these reactions. Some methods of Gaussian 09W computational package were used, such as AM1 semi-empirical method, HF (Hartree-Fock) ab initio method, B3LYP, from density functional theory (DFT) and the compound methods CBS-QB3 (CBS family) and G2 (Gaussian-n family). Two group contribution methods were also applied: Joback method and Benson method. The results of these methods were compared between each other and between experimental values, considering the accuracy and computational cost of each method. The results showed that the group contribution methods have good prediction of enthalpies of formation. From calculation of the enthalpies of reaction by molecular modeling, it was found that the AM1 method has the lowest computational cost, but its accuracy is lower when compared to the experimental values of enthalpy. The HF and B3LYP computational costs are similar, but it was observed that the DFT method has a greater precision. Also, some molecules were simulated by G2 and CBS-QB3 methods and it was found an improvement in the prediction of the geometrical parameters and thermodynamic properties of molecules. The simulations showed that the esterification reactions of fatty acids are usually exothermic, and by calculating the equilibrium constants, it was observed that these reactions are favored with decrease of temperature. The molecular modeling was also used to study the transesterification reaction of oleic acid. In this case, the high computational cost committed to obtaining satisfactory results with more accurate methods. In general, molecular modeling revealed itself as an alternative for prediction of thermodynamic data, but the computational cost is a major factor when high accuracy is required.